The surgical preparation of bone endings for receiving prosthetic knee joints for a total knee replacement is generally a complex procedure, particularly when ligaments remain attached, or when osteoarthritic changes to the joint have distorted the normal, more symmetric articulation geometry of the joint or bone. In general, it is necessary to perform soft tissue balancing and numerous specially aligned cuts at the bone ends in order to install the prosthetic components with correct spacing, alignment and tensioning to prevent improper kinematics from arising as the joint rotates in use, and to avoid the occurrence of accelerated wear patterns or possible joint dislocation.
A number of bone cuts are made to effect the placement and orientation of the femoral component of the prosthesis on the bone with the appropriate joint gaps in extension and flexion. The size and shape of these two bone gaps affect final bone orientation as well as joint tensioning and clearances when the prosthesis is installed. With respect to their effect on final orientation, the flexion gap is related to internal/external orientation of the femur, while the extension gap is related to the varus/valgus orientation of the femur.
Generally, these cuts are formed so that in extension the joint gap is perpendicular to the mechanical axis of the femur, while in flexion the joint gap is such as to place the femoral component in either neutral or external rotation and achieve patellar tracking with the femoral component. Furthermore to fit the femoral component the gaps created by the bone resections in both flexion and extension should be rectangular. In flexion, the relevant natural articulation surface corresponds to the tangent plane of the posterior epicondyles, and in extension, to that of the distal epicondylar surface. However, by performing A/P cuts by reference to the posterior surfaces, there is some risk of notching the anterior cortex. Thus, many surgeons set the A/P cut positions with reference to the anterior cortex. In one surgical protocol, the fitting is done after first resecting the distal femur, drilling positioning holes for the femoral joint component positioning pins, and then placing one or more cutting blocks or other tool alignment assemblies in the positioning holes to prepare various surface cuts.
Typically this requires a number of measurement steps and cutting or fitting steps, often with additional small adjustment cuts to achieve the final bone preparation. However it is difficult to devise a jig which dependably sets the femoral alignment because landmarks may be inconsistent or obscure. In general, the surgeon must exercise judgment as the various cuts are made. Also the steps in reaching a determination will vary depending upon the initial landmarks used for setting preliminary resections, both as a matter of the surgeon's preferred procedure and as constrained by any patient-specific features or disease.
Recently, some interest has arisen in using the epicondylar axis as a guide line, either by marking its position as a reference for slight adjustment to be made during fitting, or as a primary landmark when disease or a previous arthroplasty have altered or obliterated other landmarks. When used to set internal/external rotation this provides improved balance of the collateral ligament tension between flexion and full extension. However, it can be awkward to determine the epicondylar axis, and while the clinical epicondylar prominence may be considered in advance of surgery, the sequence of steps required to harmonize preparatory cuts with the epicondylar surfaces or opposed articulation elements of the prosthesis remains complex.
Accordingly, it would be desirable to provide a tool to simplify procedures during surgery for sizing and performing preparatory bone cuts, or for setting alignment marks to prepare the bone to receive a prosthetic joint component.
It would further be desirable to provide such a tool which aligns with the epicondyles of an exposed femur and determines one or preferably several features or measurements, such as prosthesis size, centerline and A/P offset, to enable a fitting procedure with standard cutting blocks referenced to the epicondylar axis.